1. Field of the Invention
The present invention is a method for determining the state of a battery or batteries that form a battery pack. This invention has general applications in battery charging or battery evaluation systems. More specifically, this invention has applications in battery charger control systems that utilize digital signal processor (DSP) technology for reconditioning battery packs used to power appliances such as motorized medical devices.
2. Related Art
Batteries come in many different forms and many different sizes. Regardless of battery form or application, battery quality tends to diminish over time. Quite often, batteries will remain idle for long periods of time before a demand for battery power arises. Certain applications require a highly reliable source of battery power, for example, emergency lighting systems, uninterruptible power supplies (UPS), and applications in portable appliances used during time-sensitive procedures. In these cases, a user may need to assess the quality, or state of health (SOH), of a battery under load in order to predict whether, and for how long, the battery may continue to provide reliable service.
A common method for evaluating a battery is by measuring voltage at the battery terminals while the battery is under load. However, measurements taken under load conditions are known to introduce uncertainty due resistive effects from test leads, contacts, and internal components of the battery. One attempt to overcome this problem is presented in U.S. Pat. No. 6,157,171, which describes an apparatus and method for monitoring battery voltage while disconnecting or powering down the battery load. This method measures an instantaneous no-load battery voltage, and, responsive to a comparison of the measurement to a reference value, generates an appropriate charge or discharge signal. One drawback, however, is that this method fails to account for many other factors that can affect battery voltage, such as temperature fluctuation and charge cycle history. As such, this method has a limited practical use as a control system for a battery charger. When applied to an aged battery that is unable to maintain a charge, the method may produce an oscillating output of charge and discharge signals, making it unsuitable for use in predicting a battery's SOH.